A study was made of 20 rats infested by Giardia muris in which a histologic study was made of the liver, as well as of 25 patients with giardiasis and elevated alanine-aminotransferase levels. Patients with positive A or B hepatitis markers, cholelithiasis or history of drug or alcohol use were excluded. Tests of liver function and liver biopsy were performed and antiparasite therapy was given during three months of follow-up, after which the liver biopsy was repeated. Humoral alterations were compared to those of 30 patients with acute viral hepatitis (15 type A and 15 type B) over the same periods of time. In 20% of the rats, nonspecific liver lesions were found. In the patients liver enzymes and the thymol test normalized a month after treatment and serum bile acids became normal in the third month. The liver biopsy demonstrated hepatic damage in 94% of the patients (in 20 cases cell lesions and in 12 cases inflammatory lesions) which regressed in the third month, the follow-up biopsy being normal after eradication of the parasite was confirmed. The comparative study with viral hepatitis showed highly significant differences in all the variables studied during the follow-up stage. Emphasis is placed on the importance of this lesion and its differential diagnosis to prevent its progression to chronic liver disease.